1. Field of the Invention
The present invention relates to an adhesive composition relating to adhesion of plastics and to a keypad for use in push-button switches with the adhesive composition.
2. Description of the Related Art
Heretofore, active energy ray-curing adhesives that cure upon irradiation of an active energy ray, such as an ultraviolet ray or an electron beam, have high curing rates and have been used in various fields by making the best of many advantages such as improvement of productivity, efficiency in the use of resources and energy, and safety of the working environment. However, the above-mentioned adhesives having such advantages can never be said to be perfect for any possible purpose, and some problems still remain to be improved on the following points.
One problem to be improved is poor adhesion to plastics. That is, plastics have a lower surface energy state than that of metals and depending on the kind of plastics, it is necessary to perform a preliminary treatment in the bonding step in order to improve adhesion. As such a preliminary treatment, for example, a short wavelength UV cleaning or corona discharge is performed on the surface of the plastics to activate the bonding area or a solvent-based anchoring agent as a primer is coated on the surface of the plastics. If the adhesion of adhesives to the plastics could be improved without such preliminary treatments, the bonding step would be further simplified.
Another problem to be improved is that the workability of the bonding adhesive is unsatisfactory. That is, the use of conventional adhesives may result in stringing of the adhesive from the nozzle of a coating apparatus, and also may result in sagging of the adhesive after coating when coated on members or the like with an edge surface. The poor workability of the adhesive raises such problems as adhesive composition adhering to a different part relative to the target part, or the adhesive sagging and consequently aggravating the design of the product as members to be bonded at positions adjacent to each other cannot be bonded at right positions, respectively.
The two problems to be improved as described above will now be described more specifically. A keypad for push-button switches for use in inputting operations is utilized in various articles such as cellular phones, personal digital assistance, remote controls for various household electrical appliances, card remote controls, and a variety of keyboards. The keypad for push-button switches has a structure that includes a base portion made of a rubbery elastic material such as silicone rubber, and a keytop portion made of molded hard resin such ABS resin and fixingly bonded to the base portion with an adhesive. The keytop portion is marked with a character, a numeric character or a symbol as an input element. Pressing the keytop portion makes the base portion warp in the direction of the pressing so that an electroconductive contact point positioned below the base portion is brought into electrical continuity, thus allowing a predetermined input of the marked character or the like.
In such a keypad for push-button switches, the keytop portion is thus bonded to the base portion with an adhesive and in the step of bonding it, the above-mentioned two problems still remain to be solved. That is, the keypad for push-button switches is subject to being repeatedly pressed. Accordingly, the adhesive layer must have high adhesion so that it can endure the force of the pressing operation repeatedly applied thereto. However, the above-mentioned conventional adhesives cannot provide such high adhesion and it is necessary to subject the keytop portion or base portion to the above-mentioned preliminary treatments before they can be bonded in the bonding step in order to increase the adhesion. Therefore, it has been practically difficult to simplify the bonding step.
Further, since the above-mentioned conventional adhesives have poor bonding workability, the adhesive composition may adhere to a different part relative to the target part, or sags and consequently aggravates the design of the keypad for push-button switches. On the other hand, in the case where keytop portions must be bonded at positions adjacent to each other, for example, as in the case of the keypad for push-button switches in a cellular phone, the keytop portions cannot be bonded at precise positions, respectively.
The inventors of the present invention have made extensive studies with a view toward solving the above-mentioned problems and as a result, they have found that a specified adhesive composition is useful as the above-mentioned active energy ray-curing adhesive composition and is excellent in overcoming the above-mentioned problems relating to adhesion and bonding workability, thereby achieving the present invention.
A first object of the present invention is to provide an active energy ray-curing adhesive composition that not only has a high curing rate but also is excellent in adhesion and bonding workability.
A second object of the present invention is to provide a keypad for a push-button switch in which not only the bonding speed of a keytop portion to a base portion is high but also the adhesion and bonding workability during a bonding step are excellent.
More particularly, the present invention relates to an active energy ray-curing adhesive composition containing a urethane (meth)acrylate, a (meth)acrylamide compound, and a hydroxyl group-containing (meth)acrylate.
The active energy ray-curing adhesive composition has good adhesion to plastics. The (meth)acrylamide compound containing a component having a dialkylacrylamide group, in particular, N,N-diethylacrylamide, is preferable since the adhesive force to plastics becomes higher. Further, an adhesive composition containing a particulate silicic anhydride and at least one of a tertiary amine salt or a tertiary amine in addition to the urethane (meth)acrylate, the (meth)acrylamide compound, and the hydroxyl group-containing (meth) acrylate, does not cause stringing from the nozzle of a coating apparatus, shows less sagging when it is coated, and can be coated accurately, sufficiently and easily on a bonding area.
Further, the present invention provides a keypad for a push-button switch including a base portion made of a rubbery elastic material and a keytop portion made of a hard resin and fixed to the base portion, in which the keytop portion is bonded to the base portion through an adhesive layer made of a solidified active energy ray-curing adhesive composition containing a urethane (meth)acrylate, a (meth)acrylamide compound, and a hydroxyl group-containing (meth)acrylate.
According to this keypad for the push-button switch, high adhesive strength is obtained and it is difficult for the keytop portion to be peeled away from the base portion since the base portion made of a rubbery elastic material and the keytop portion made of a hard resin are bonded together through the adhesive layer made of the solidified active energy ray-curing adhesive composition containing a urethane (meth) acrylate, a (meth) acrylamide compound, and a hydroxyl group-containing (meth)acrylate. In particular, when the (meth)acrylamide compound contains a component having a dialkylacrylamide group, particularly N,N-diethylacrylamide, the keypad is excellent in improvement of adhesion.
As components to be contained in the adhesive composition that constitutes the adhesive layer, besides the urethane (meth) acrylate, the (meth)acrylamide compound, and the hydroxyl group-containing (meth)acrylate, a particulate silicic anhydride, and at least one of a tertiary amine salt or a tertiary amine may be contained. Use of an adhesive composition that contains such components makes it possible to obtain a keypad for a push-button switch that causes neither stringing from the nozzle of a coating apparatus nor sagging after coating when the keytop portion made of a hard resin and the base portion made of a rubbery elastic material are bonded to each other, that has excellent bonding operationality and excellent bonding workability, and that can be produced easily and at a low cost.
The keypad for the push-button switch, in which the above-mentioned active energy ray-curing adhesive composition is an ultraviolet ray-curing adhesive composition, allows production in a relatively short time since the solidification of the adhesive composition can take place in a time on the order of second. In addition, stability during coating operation or during storage is high and there is no need to mix plural compositions before use, so that the workability is excellent. Therefore, the production step becomes simpler and faster and hence inexpensive keypads for push-button switches can be obtained.
Further, even if a protective layer that protects the base portion is formed between the base portion and the adhesive layer, it is difficult for the keytop portion to be peeled away from the base portion since the adhesion between the protective layer and the adhesive layer made of the above-mentioned solidified adhesive composition is excellent. In particular, in the case where the protective layer is made of a crosslinking urethane resin, the keypad has a disadvantage. That is, although the resin has not been crosslinked at the time of coating, thus giving excellent workability, and being exceedingly durable due to crosslinking, a phenomenon occurs in which the initial adhesion changes with a lapse of time, presumably since the reaction gradually proceeds also after the formation of the keypad, and thus the keypad after a considerable time is not sufficiently adhered. However, according to the present invention, a decrease in adhesion does not occur even in such a resin and the adhesive retains sufficient adhesive force even with a lapse of time. Therefore, even in the case of a composite keypad including a flexible keytop portion formed on the base portion made of a rubbery elastic material such as silicone rubber, by providing the base portion with a partially thick protrusion and a hard keytop portion made of a hard resin fixed to the base portion with an adhesive, the keytop portion made of a hard resin is firmly bonded to the base portion, so that a keypad for push-button switches that is difficult to be peeled from the base portion can be obtained.
The present invention is not limited to what has been described above and the objects, advantages, features and utilities of the present invention will become clearer by the following description with reference to the accompanying drawings. Also, it should be understood that the appropriate variations of the described invention, made without departing from the scope and spirit of the present invention be encompassed within the scope of the invention as claimed.